Steric sea level changes estimated from historical ocean subsurface temperature and salinity analyses

An historical objective analysis of subsurface temperature and salinity was carried out on a monthly basis from 1945 to 2003 using the latest observational databases and a sea surface temperature analysis. In addition, steric sea level changes were mainly examined using outputs of the objective analyses. The objective analysis is a revised version of Ishii et al. and is available at 16 levels in the upper 700 m depth. Artificial errors in the previous analysis during the 1990s have been worked out in the present analysis. The steric sea level computed from the temperature analysis has been verified with tide gauge observations and TOPEX/Poseidon sea surface height data. A correction for crustal movement is applied for tide gauge data along the Japanese coast. The new analysis is suitable for the discussion of global warming. Validation against the tide gauge reveals that the amplitude of thermosteric sea level becomes larger and the agreement improves in comparison with the previous analysis. A substantial part of local sea level rise along the Japanese coast appears to be explained by the thermosteric effect. The thermal expansion averaged in all longitudes from 60°S to 60°N explains at most half of recent sea level rise detected by satellite observation during the last decade. Considerable uncertainties remain in steric sea level, particularly over the southern oceans. Temperature changes within MLD make no effective contribution to steric sea level changes along the Antarctic Circumpolar Current. According to statistics using only reliable profiles of the temperature and salinity analyses, salinity variations are intrinsically important to steric sea level changes in high latitudes and in the Atlantic Ocean. Although data sparseness is severe even in the latest decade, linear trends of global mean thermosteric and halosteric sea level for 1955 to 2003 are estimated to be 0.31 ± 0.07 mm/yr and 0.04 ± 0.01 mm/yr, respectively. These estimates are comparable to those of the former studies.     

Absolute sea level variability of Arctic Ocean in 1993–2018 from satellite altimetry and tide gauge observations

Arctic absolute sea level variations were analyzed based on multi-mission satellite altimetry data and tide gauge observations for the period of 1993–2018. The range of linear absolute sea level trends were found ?2.00 mm/a to 6.88 mm/a excluding the central Arctic, positive trend rates were predominantly located in shallow water and coastal areas, and negative rates were located in high-latitude areas and Baffin Bay. Satellite-derived results show that the average secular absolute sea level trend was (2.53±0.42) mm/a in the Arctic region. Large differences were presented between satellite-derived and tide gauge results, which are mainly due to low satellite data coverage, uncertainties in tidal height processing and vertical land movement (VLM). The VLM rates at 11 global navigation satellite system stations around the Arctic Ocean were analyzed, among which 6 stations were tide gauge co-located, the results indicate that the absolute sea level trends after VLM corrected were of the same magnitude as satellite altimetry results. Accurately calculating VLM is the primary uncertainty in interpreting tide gauge measurements such that differences between tide gauge and satellite altimetry data are attributable generally to VLM.     

Interannual Sea Level Variations in the South China Sea Over 1950–2009

Spatial patterns of interannual sea level variations in the South China Sea (SCS) are investigated by analyzing an EOF-based 2-dimensional past sea level reconstruction from 1950 to 2009 and satellite altimetry data from 1993 to 2009. Long-term tide gauge records from 14 selected stations in this region are also used to assess the quality of reconstructed sea levels and determine the rate of sea level along the coastal area. We found that the rising rate of sea levels derived from merged satellite altimetry data during 1993–2009 and past sea level reconstruction over 1950–2009 is about 3.9 ± 0.6 mm/yr and 1.7 ± 0.1 mm/yr, respectively. For the longer period, this rate is not significantly different from the global mean rate (of 1.8 ± 0.3 mm/yr). The interannual mean sea level of the SCS region appears highly correlated with Niño 4 indices (a proxy of El Niño-Southern Oscillation/ENSO), suggesting that the interannual sea level variations over the SCS region is driven by ENSO events. Interpolation of the reconstructed sea level data for 1950–2009 at sites where tide gauge records are of poor quality (either short or gapped) show that sea level along the Chinese coastal area is rising faster than the global mean rate of 1.8 mm/yr. At some sites, the rate is up to 2.5 mm/yr.     

Steric height trends at Ocean Station Papa in the Northeast Pacific Ocean

Abstract

We estimate secular changes in steric sea level in the northeast Pacific Ocean using the 27‐year time series of monthly hydrographic observations for Station PAPA (50°N, 145°W). Linear trends based on the entire data record suggest that steric heights relative to 1000 db are increasing at a rate of 0.93 mm/yr and that 67% of this increase is due to thermosteric changes at depths below 100 m; the smaller halosteric contribution to the steric trend appears to be confined to the upper 100 m. A trend of 0(1 mm/yr) is consistent with estimates of sea level rise based on coastal tide gauge records. However, a critical examination of the results indicates that sea level changes of such small magnitude would be masked by the large (1–10 cm) interannual variability of open ocean steric height. This is verified by recalculation of trends using abridged versions of the data set. We conclude that our trend estimates are still open to question and that the present 27‐year time series is too short to permit accurate resolution of possible climate‐induced changes in global sea level.     

Intraannual variability of sea level around Tosa Bay

Through analysis of monthly in situ hydrographic, tide gauge, altimetry and Kuroshio axis data for the years 1993–2001, the intraannual variability of sea level around Tosa Bay, Japan, with periods of 2–12 months is examined together with the intraannual variability of the Kuroshio south of the bay. It is shown that the intraannual variation of steric height on the slope in Tosa Bay can account for that of sea level at the coast around the bay as well as on this slope. It is found that the steric height (or sea level) variation on the slope in this bay is mainly controlled by the subsurface thermal variation correlated with the Kuroshio variation off Cape Ashizuri, the western edge of Tosa Bay. That is, when the nearshore Kuroshio velocity south of the cape is intensified [weakened] concurrently with the northward [southward] displacement of the current axis, temperature in an entire water column decreases [increases] simultaneously, mainly due to the upward [downward] displacement of isotherms, coincident with that of the main thermocline. It follows that the steric height (or sea level) decreases [increases].     

1993—2016年中国近海海平面时空变化趋势

区域海平面变化是目前气候变化研究的热点问题.海平面变化具有时间和空间的异质性,分析海平面变化,应充分考虑时间和空间的差异.基于集合经验模态分解(Ensemble Empirical Mode Decomposition,EEMD)、最小二乘法,利用卫星高度计、验潮站数据,分析了1993—2016年间中国近海及周边海域海...     

Estimating trends of the Mediterranean Sea level changes from tide gauge and satellite altimetry data (1993-2015)

The impact of climate change on sea level has received a great deal of attention by scientists worldwide. In this context, the problem of sea levels on global and regional scales have been analyzed in a number of studies based on tide gauges observations and satellite altimetry measurements. This study focuses on trend estimates from 18 high-quality tide gauge stations along the Mediterranean Sea coast. The seasonal Mann-Kendall test was run at a 5% significance level for each of the 18 stations for the period of 1993-2015 (satellite altimetry era). The results of this test indicate that the trends for 17 stations were statistically significant and showed an increase (no significant trend was observed only at one station). The rates of sea level change for the 17 stations that exhibit significant trends, estimated using seasonal Sen's approach, range after correction for Vertical Land Motion (VLM) from 1.48 to 8.72 mm/a for the period 1993-2015. Furthermore, the magnitude of change at the location of each tide gauge station was estimated using the satellite altimetry measurements. Thus, the results obtained agree with those from the tide-gauge data analysis.     

Inferring the global mean sea level from a global tide gauge network

An attempt is made to infer the global mean sea level(GMSL) from a global tide gauge network and frame the problem in terms of the limitations of the network. The network,owing to its limited number of gauges and poor geographical distribution complicated further by unknown vertical land movements,is ill suited for measuring the GMSL. Yet it remains the only available source for deciphering the sea level rise over the last 100 a. The poor sampling characteristics of the tide gauge network have necessitated the usage of statistical inference. A linear optimal estimator based on the Gauss-Markov theorem seems well suited for the job. This still leaves a great deal of freedom in choosing the estimator. GMSL is poorly correlated with tide gauge measurements because the small uniform rise and fall of sea level are masked by the far larger regional signals. On the other hand,a regional mean sea level(RMSL) is much better correlated with the corresponding regional tide gauge measurements. Since the GMSL is simply the sum of RMSLs,the problem is transformed to one of estimating the RMSLs from regional tide gauge measurements. Specifically for the annual heating and cooling cycle,we separate the global ocean into 10-latitude bands and compute for each 10-latitude band the estimator that predicts its RMSL from tide gauges within. In the future,the statistical correlations are to be computed using satellite altimetry. However,as a first attempt,we have used numerical model outputs instead to isolate the problem so as not to get distracted by altimetry or tide gauge errors. That is,model outputs for sea level at tide gauge locations of the GLOSS network are taken as tide gauge measurements,and the RMSLs are computed from the model outputs. The results show an estimation error of approximately 2 mm versus an error of 2.7 cm if we simply average the tide gauge measurements to estimate the GMSL,caused by the much larger regional seasonal cycle and mesoscale variation plaguing the individual tide gauges. The numerical model,Los Alamos POP model Run 11 lasting 3 1/4 a,is one of the best eddy-resolving models and does a good job simulating the annual heating and cooling cycle,but it has no global or regional trend. Thus it has basically succeeded in estimating the seasonal cycle of the GMSL. This is still going to be the case even if we use the altimetry data because the RMSLs are dominated by the seasonal cycle in relatively short periods. For estimating the GMSL trend,longer records and low-pass filtering to isolate the statistical relations that are of interest. Here we have managed to avoid the much larger regional seasonal cycle plaguing individual tide gauges to get a fairly accurate estimate of the much smaller seasonal cycle in the GMSL so as to enhance the prospect of an accurate estimate of GMSL trend in short periods. One should reasonably expect to be able to do the same for longer periods during which tide gauges are plagued by much larger regional interannual(e. g.,ENSO events) and decadal sea level variations. In the future,with the availability of the satellite altimeter data,we could use the same approach adopted here to estimate the seasonal variations of GMSL and RMSL accurately and remove these seasonal variations accordingly so as to get a more accurate statistical inference between the tide gauge data and the RMSLs(therefore the GMSL) at periods longer than 1 a,i. e.,the long-term trend.     

Vertical Land Motion in the Mediterranean Sea from Altimetry and Tide Gauge Stations

We have computed estimates of the rate of vertical land motion in the Mediterranean Sea from differences of sea level heights measured by the TOPEX/Poseidon radar altimeter and by a set of tide gauge stations. The comparison of data at 16 tide gauges, using both hourly data from local datasets and monthly data from the PSMSL dataset, shows a general agreement, significant differences are found at only one location. Differences of near-simultaneous, monthly and deseasoned monthly sea level height time-series have been considered in order to reduce the error in the estimated linear-term. In a subset of 23 tide gauge stations the mean accuracy of the estimated vertical rates is 2.3 ± 0.8 mm/yr. Results for various stations are in agreement with estimates of vertical land motion from geodetic methods. A comparison with vertical motion estimated by GPS at four locations shows a mean difference of -0.04 ± 1.8 mm/yr, however the length of the GPS time-series and the number of locations are too small to draw general conclusions.     

联合卫星测高和卫星重力数据研究热容海平面变化

首先用卫星测高资料计算了1993～2009年6月的全球平均海平面变化。用GRACE(gravity recovery andclimate experiment)时变重力场系数反演了2003～2009年6月全球平均海水质量变化。联合GRACE和卫星测高资料计算了2003～2009年6月的热容海平面变化,该变化呈上升趋势。用日本气象局Ishii等提供的海温数据计算了1993～2006年的海水引起的平均热膨胀海平面变化,1993～2003年间,全球海洋热膨胀引起的热容海平面呈上升趋势,约占同期平均海平面变化的一半。利用ARGO温盐数据计算了2004～2009年6月平均热容海平面变化,也呈上升态势,只是变化速率有所减慢。     

Recent sea level trends and accelerations: Comparison of tide gauge and satellite results

A comprehensive set of 456 monthly tide gauge records is analyzed for trend and acceleration over the same period that satellite altimetry was analyzed (1993 to 2011). Additionally, a 90 tide gauge record subset is analyzed for which GPS data are available. The selection criterion for the tide gauge data is 85% data completion. All measurements are adjusted for vertical land motion. Results from 456 pairs of tide gauges, adjusted for Global Isostatic Adjustment, and satellite recordings located within 1° root-mean-square latitude and longitude separation differences are compared. The tide gauge trends and accelerations are adjusted for spatial bias using the more globally dense satellite data.The average trends of the 456 and 90 gauge sets (3.26 and 2.68 mm/year, respectively) agree reasonably well with the global trend average of the satellite data (3.09 mm/year). Average trends for the 456 tide gauges are also in good agreement (within 95% confidence limits) with trends based on satellite data within the 1° satellite proximity criterion (3.26 and 3.31 mm/year, respectively). The trends for the 90 gauges with GPS nearby and qualifying satellite locations are 2.68 and 2.74 mm/year, respectively. For all datasets analyzed, the accelerations are quite strongly negative but the uncertainty is relatively large. Adjustment of the tide gauge trends for spatial bias modified both trends and accelerations significantly and decreased trend differences between the 456 and 90 gauge datasets. The spatially adjusted tide gauge trends (2.95 and 2.72 mm/year, respectively for the 456 and 90 tide gauges sets) are somewhat less than the 1° spatially adjusted satellite data (3.09 mm/year). Whether the increased sea level trend of approximately 3 mm/year measured by the satellites since the 1990's is a long-term increase from the 20th Century value of approximately 1.7 mm/year or part of a cycle will require longer records; however, the negative accelerations support some cyclic character.     

Validation of a decadal OGCM simulation for the tropical Pacific

An ocean general circulation model is forced with NCEP reanalysis over the 1948–1999 period. The simulated dynamic height and sea level are compared respectively to the dynamic height computed from hydrological data and to the sea level measured by tide gauges in the tropical Pacific. The model is shown to capture important features of the temporal structure of variability in the tide gauge data over the last several decades. However, the comparison reveals a largely artificial trend in the simulation, which consists of a decreasing dynamic height and sea level in the southwest and northwest of the tropical Pacific. Model sensitivity experiments show this trend is controlled by the NCEP surface wind stresses and more precisely by a weakening in the trade winds and a trend in the off-equatorial wind curl, with this trend existing mainly before the mid 1970s. For studies of decadal variability, the simple removal of a linear trend is an inadequate way to solve the problem, due to the inhomogeneities in the data used in reanalysis products and the non-linearity of models.     

Vertical Land Motion in the Mediterranean Sea from Altimetry and Tide Gauge Stations

We have computed estimates of the rate of vertical land motion in the Mediterranean Sea from differences of sea level heights measured by the TOPEX/Poseidon radar altimeter and by a set of tide gauge stations. The comparison of data at 16 tide gauges, using both hourly data from local datasets and monthly data from the PSMSL dataset, shows a general agreement, significant differences are found at only one location. Differences of near-simultaneous, monthly and deseasoned monthly sea level height time-series have been considered in order to reduce the error in the estimated linear-term. In a subset of 23 tide gauge stations the mean accuracy of the estimated vertical rates is 2.3 ± 0.8 mm/yr. Results for various stations are in agreement with estimates of vertical land motion from geodetic methods. A comparison with vertical motion estimated by GPS at four locations shows a mean difference of ?0.04 ± 1.8 mm/yr, however the length of the GPS time-series and the number of locations are too small to draw general conclusions.     

IPCC气候情景下全球海平面长期趋势变化

利用CCSM3 (Community Climate System Model version 3)气候系统模式模拟20世纪海平面变化,在IPCC SRES A2 (IPCC,2001)情景假设下预测21世纪全球海平面长期趋势变化。模拟显示20世纪海平面上升约4.0 cm,且存在0.004 8 mm/a2的加速度,这个结果仅为热盐比容的贡献。在A2情景假设下,21世纪海平面上升存在很大的区域特征,呈纬向带状分布;总体上北冰洋上升大,南大洋高纬度海区上升小,大西洋上升值比太平洋的大;整个21世纪全球平均比容海平面上升了约30 cm,且呈加速上升的趋势。同时发现,中深层水温度和盐度变化对区域比容海平面变化具有重要贡献。北太平洋增暖主要集中在上层700 m以内,而北大西洋的增暖可达2 500 m的深度,南大洋南极绕极流海区热盐变化则是发生在整个深度。     

Improving the Coastal Mean Dynamic Topography by Geodetic Combination of Tide Gauge and Satellite Altimetry

Abstract

The ocean mean dynamic topography (MDT) is the surface representation of the ocean circulation. The MDT may be determined by the ocean approach, which involves temporal averaging of numerical ocean circulation model information, or by the geodetic approach, wherein the MDT is derived using the ellipsoidal height of the mean sea surface (MSS), or mean sea level (MSL) minus the geoid as the geoid. The ellipsoidal height of the MSS might be estimated either by satellite or coastal tide gauges by connecting the tide gauge datum to the Earth-centred reference frame. In this article we present a novel approach to improve the coastal MDT, where the solution is based on both satellite altimetry and tide gauge data using new set of 302 tide gauges with ellipsoidal heights through the SONEL network. The approach was evaluated for the Northeast Atlantic coast where a dense network of GNSS-surveyed tide gauges is available. The typical misfit between tide gauge and satellite or oceanographic MDT was found to be around 9?cm. This misfit was found to be mainly due to small scale geoid errors. Similarly, we found, that a single tide gauge places only weak constraints on the coastal dynamic topography.     

Sea level rise along China coast in the last 60 years

Based on long-term tide gauge observations in the last 60 years, the temporal and spatial variation characteristics of sea level change along the coast of China are analyzed. The results indicate that the sea level along the coast of China has been rising at an increasing rate, with an estimated acceleration of 0.07 mm/a2. The rise rates were 2.4 mm/a, 3.4 mm/a and 3.9 mm/a during 1960–2020, 1980–2020 and 1993–2020, respectively. In the last 40 years, the coastal sea level has risen fastest in the South China Sea and slowest in the Yellow Sea. Seasonal sea levels all show an upward trend but rise faster in winter and spring and slower in autumn. Sea level change along the coast of China has significant periodic oscillations of quasi-2 a, 4 a, 7 a, 11 a, quasi-19 a and 30–50 a, among which the 2–3 a, 11 a, and 30–50 a signals are most remarkable, and the amplitude is approximately 1–2 cm. The coastal sea level in the most recent decade reached its highest value in the last 60 years. The decadal sea level from 2010 to 2019 was approximately 133 mm higher than the average of 1960–1969. Empirical orthogonal function analysis indicates that China’s coastal sea level has been changing in a north-south anti-phase pattern, with Pingtan and Fujian as the demarcation areas. This difference was especially obvious during 1980–1983, 1995–1997 and 2011–2013. The coastal sea level was the highest in 2016, and this extreme sea level event was analyzed to be related mainly to the anomalous wind field and ENSO.     

赤湾验潮站搬迁前后潮位变化特征及其订正

赤湾海洋站是国家海洋局在深圳西部海域唯一的潮汐观测站点,至今已观测30多年。由于地理位置特殊,2008年起该站被选为《中国海平面公报》反映珠江口海平面变化的代表站。2012年10月该站搬至新验潮站,受此影响,其潮位观测序列产生了差异性和不连续性。本文通过计算机编程实现了对海洋站水文气象数据的质量控制。同时,采用赤湾站与邻近验潮站进行对比的方法得出新旧站的潮位差值,初步探讨了搬站前后潮位观测结果的订正方案,并研究其建站以来的历史海平面变化特征。这对确保该站潮位资料的准确性及代表性具有重要意义,同时也为珠江口海域防灾减灾、涉海工程、风暴潮预报等提供了数据支撑。     

验潮井注入防冻油层对潮位观测影响分析

在全球气候变暖的影响下,中国沿海的海平面上升趋势还将进一步加剧带来海水倒灌、沿海地区水质恶化、生态环境和资源破坏、侵蚀海岸、海洋自然灾害发生的频率增高等一系列危害。为缓解这些危害,首先要掌握海平面的上升速率。潮位观测数据是对海平面进行科学研究的重要依据之一,潮位数据的及时性、准确性、完整性在海平面研究中起主导作用。近年来北方冬季海面结冰现象严重,如不采取相应措施会导致验潮井内结冰,潮位数据中断,从而无法计算平均海平面高度。为防止北方冬季验潮井内结冰,常向验潮井内注入防冻柴油,但注入柴油后,验潮井内液面与外海高度不同,造成潮位观测不准确。根据潮位观测理论和多年实践经验,计算出在不影响潮位读数的情况下,可注入防冻油层的最大高度,并对超过最大高度时潮位读数的修正数值进行了研究。     

The Role of GNSS Vertical Velocities to Correct Estimates of Sea Level Rise from Tide Gauge Measurements in Greece

In this study, we show how the Global Navigation Satellite System (GNSS)-derived vertical velocities contribute to the correction of tide gauge (TG) measurements used for the sea level rise estimation in Greece. Twelve sites with records of local sea level heights are processed in order to estimate their trend. Certain error sources related to TGs, e.g. equipment changes, data noise, may lead to biased or erroneous estimations of the sea level height. Therefore, it would be preferred to follow a robust estimation technique in order to detect and reduce outlier effects. The geocentric sea level rise is estimated by taking into account the land vertical motion of co-located GNSS permanent stations at the Hellenic area. TGs measure the height of the water relative to a monitored geodetic benchmark on land. On the other hand, using GNSS-based methods the vertical land motion can be derived. By means of extended models fitted to the GNSS time-series position, obtained from seven years of continuous data analysis, periodic signals are well described. The synergy of the two co-located techniques results in the correction of TG relative sea level heights taking into account the GNSS vertical velocities and consequently obtaining the conversion to absolute (geocentric) sea level trend.     

中国主要入海河口咸潮入侵变化特征

长江口、钱塘江口和珠江口是受咸潮影响较为严重的区域。本文利用全国沿海海平面变化影响调查、沿海水文观测等数据,分析了近十年长江口、珠江口和钱塘江口咸潮入侵的变化特征及影响。分析结果表明:(1) 2009-2018年,长江口咸潮入侵次数和持续时间均呈减少趋势,该时段长江口共监测到约48次咸潮入侵过程,发生时间集中在9-10月至翌年5月,其中3月和11月入侵次数较多,分别为12次和7次。(2)钱塘江口咸潮入侵过程受沿海季节性海平面影响显著,12月至翌年3月为钱塘江口季节性低海平面期,4-7月上旬径流量较大,上述两个时期钱塘江口受咸潮入侵的影响均较小,7月下旬至11月上旬,钱塘江口处于季节性高海平面期,是咸潮影响的集中时段。(3) 2009-2018年,珠江口共监测到约57次咸潮入侵过程,发生时间集中在9-10月至翌年3-4月,其中1月、2月和10月咸潮入侵次数较多,均超过10次,2015年至今咸潮持续时间明显增加。(4)咸潮入侵次数和持续时间与基础海面和径流量等密切相关,咸潮入侵影响三大河口沿线水厂供水以及工农业生产取水,给沿岸城市的居民生活、工农业生产和渔业养殖等造成一定不利影响。